The present invention relates to liquid rocket engines and specifically re-usable rocket engines which are operated many times.
Most liquid rocket engines have been expendable. That is, when the engine design reaches operational maturity, each production engine is operated probably only twice—once in a ground test to verify that the engine works and a second time when it is flown and expended. The only re-usable liquid rocket engine in operation today is the Space Shuttle Main Engine.
Several U.S. liquid rocket engine combustion chambers are made using tubes. The tubes are bent into the shape of the rocket chamber/nozzle, which is similar to a slice through the familiar “bell” shape. Then the tubes are fixtured in an axisymmetric array and joined together to form the complete chamber/nozzle. To complete the chamber/nozzle, external structure must be added to strengthen the joined tubes, and manifolds must be added at each end of the array of tubes for coolant inlet and discharge. The advantage of a tube is that it is an excellent pressure vessel. The disadvantages however are numerous. Each individual tube must be formed to size and shape to conform to design specifications. Also, it takes a large number to make a complete chamber, in many cases 200 or more. This makes the tubes, essentially the chamber/nozzle raw material, a large cost item. Next the tubes and manifolds must be joined in place without leaks. This is very difficult to do without fault, and re-work is often required, further increasing chamber/nozzle cost.
Some liquid rocket engines build the chamber/nozzle in a different way. This method is known as “milled channel with close-out”. First, a “bell” is made beginning with flat sheet and rolling/welding or some similar process. Then coolant channels are machined into the bell shaped structure. A second bell which fits over the first one is then made. The second bell is then joined to the first to create the channel close-outs. In similarity with the tubular construction described above, structural stiffeners and inlet/discharge manifolds are then joined in place to complete the chamber. Advantages are that this method can be made to work with fewer parts than with tubes. Disadvantages include the operations used to join different parts. Also, the resulting channel structure is rectangular, with one of the flat walls facing the combustion chamber. The rectangular passage, and especially the flat side of the passage facing the combustion chamber, undergoes plastic deformation while operating. This results in actual thinning of the wall which does not recover original thickness on shutdown. Thinning increases with subsequent use. This phenomena has been termed “ratcheting” by the structures analysts. Overall result is a life-limited structure, with life nominally less than 100 cycles. Tubular chamber construction undergoes similar operational structural deformation, however, not to the same degree as with the milled channel construction, primarily due to the tube acting as an efficient pressure vessel.
An advanced method has been developed to make liquid rocket chambers and nozzles. Specifically, the method is known as vacuum plasma spray and is exemplified by U.S. Pat. No. 5,249,357. This method is equivalent to the milled channel method described above with two primary differences: (1) the “bell” is sprayed on a bell-shaped mandrel beginning with powder raw material and using a high-temperature inert gas stream; and (2) the channel close-outs are similarly sprayed. At this point in time, the method has been used to make only flat wall chambers. These have rectangular cooling channels with one of the flat walls facing the combustion chamber. Fabrication costs are significantly lower than either tubular construction or milled channel with joined close-out, due primarily to using the spray process to create a near net shape, integral (one-piece) chamber/nozzle. The disadvantage of this method is the flat wall facing the combustion chamber and its life-limiting characteristics of deflection and thinning at operation pressure and temperature.